The present invention relates to guided wave radar measurement used to detect the level of a fluid within an enclosed tank.
A guided wave radar tank probe is inserted into a tank from the top and submerged into process fluids. The probe is then used to send guided electromagnetic waves into the fluid or fluids contained within the tank. Electromagnetic waves are sent through the probe by a transmitter. Reflection occurs when the waves encounter a fluid with a different dielectric constant. At this point, part of the energy will be reflected back in the form of a reflected wave. The un-reflected portion of the wave will continue through the fluid until it reaches another fluid with a different dielectric constant, the bottom of the tank, or the end of the probe. All of the reflections are received by a receiver.
Reception occurs after waves are reflected back by the surface of the fluids within the tank. The speed of the wave and the strength of the wave reflection are dependent on the dielectric constant of the fluid(s) through which the wave travels. After receipt, the reflection or reflections are processed for timing differentials. These timing differentials are converted to a distance and the level of fluid contained within the tank is then calculated.
Several factors, such as errant transmission or reception of the electromagnetic wave, improper installation of wires or connecting pipes, a change in tank pressure or temperature, a change in tank fluid flow rate, a change in fluid concentration, a change in fluid density, or the measuring of level of fluids with a very low dielectric constant (such as oil which has a dielectric constant that is about eighty times less than water) may influence the accuracy of the level calculation. An inaccurate calculation causes incorrect reporting of fluid levels within the tank, which may lead to false alarms and unnecessary stoppages in the process for which the tank is being used.
Attempts have been made to overcome level inaccuracies. Radar systems that send multiple radar signals through one or multiple transceivers have been used to calculate multiple tank levels for comparison. However, these systems, while good at detecting failure of one radar component, are still susceptible to other distortion-causing system factors described above.
Further attempts at overcoming radar inaccuracy have been made by adding individual, non-integral pressure sensors at fixed locations. However, these sensors are limited in accuracy and usefulness by their placement in the tank. This solution is also difficult and expensive to install because the sensors are external to the probe assembly, requiring additional tank modifications.